Promotion runs today and Tuesday only

Microsoft on Monday kicked off a two-day promotion that gives college students Office 365 for up to six months free of charge.

The deal is the first trial offered for Office 365 University, a four-year subscription plan that lets eligible university and college students, faculty and staff install Office 2013 on up to two PCs and Macs, as well as install Office Mobile Apps on two smartphones running Windows Phone 7.5 or later. The plan's list price: $79.99.

Today's promotion is for U.S. students only, Microsoft said in a blog post and in the trial's terms.

Along with the three months, students receive an additional 20GB of storage space on SkyDrive, Microsoft's cloud-based service, atop the usual 7GB all customers are allotted.

The three-month trial is triple the length of a similar try-before-you-buy that Microsoft offers for Office 365 Home Premium.

Students can extend the trial to six months, said Microsoft, by sharing on Facebook. "To share via Facebook, the user must visit the Offer Site, enter and submit their qualifying .edu email address, then click the share link, log into their Facebook account and share the Offer," the terms stated.

Microsoft will shut off the promotional spigot at 11:59 p.m. PT Tuesday, March 12, or "while supplies last." Since there are no physical goods involved -- Office 365 delivers copies of Office 2013 via download -- the latter implies Microsoft has set a cap on the number of allowed trials.

Like Office 365 Home Premium, the University plan cannot be used for commercial purposes. It includes Access, Excel, OneNote, Outlook, PowerPoint, Publisher and Word on Windows; and Excel, Outlook, PowerPoint and Word on OS X.

As with any Office 365 plan, if the student decides not to convert the University trial into a paying subscription, the Office 2013 applications revert to a limited-function mode -- existing documents can be viewed and printed, but new ones cannot be created and ones already generated cannot be edited -- and the SkyDrive storage space slips back to 7GB.